Automatic gain control circuit



Oct. H. M.- N'EUSTADT I AUTOMATIC GAIN'CONTROL CIRCUIT Filed Oct. 7, 1959 0Ercr'0/2 704E NETWORK A V6 RECTIFIER roe/2115s 0F PR'To'R' mess I T Am 24 2 2&5 NETWORK I 10 I 33 5 l r0 POWER RECTIFIER T0]. AMPL. T 7'0 6/3/08 0F i CONTROLLED TUBES 1 AVC , INVENTOR. HERBFJM. NEUSTADT ATTORNEY.

Patented Oct. 21, 1941 AUTOMATIC GAIW CONTROL CIRCUIT Herbert M. Neustadt, Nutley, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application October 7, 1939, Serial No. 298,357

, 8 Claims.

My present invention relates to gain control circuits, and more particularly to such circuits when utilized for automatic volume control of radio receivers.

One of the main objects of the present invention is to provide automatic gain control circuits for signal transmission amplifiers, and which circuits embody tubes which are provided with an electrode section adapted for signal detection and another electrode section adapted to be energized by alternating current energy for the production of gain control voltage.

Another important object of this invention is to provide automatic volume controlcircuits for radio receivers wherein the automatic volume control tube may have a cold electrode thereof energized by alternating current energy, the alternating current energy being derived from an existent source in the receiver,v such as the tunable local oscillator circuit of a superhetero' dyne receiver, or the power rectifier of the receiver; and certain electrodes of the Volume control tube, additionally, functioning to detect high frequency signal carrier energy to provide the audio voltage of the receiver. l

Still other objects of the invention are to pro-.- vide a detector tube for a radio receiver wherein the tube includes a plurality of electrodes functioning as a rectifier of alternating current energy derived from the power rectifier of the receiver thereby to produce uni-directional gain control voltage for a signal transmission tube preceding the detector.

Still other objects of the invention are to improve generally the efiiciency of automatic gain control circuits for radio receivers, and more especially to provide in radio receivers automatic volume control circuits which are not only convenient and reliable in operation, but are readily assembled in a receiver.

The novelty features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing-- Fig. 1 diagrammaticallyshows a portion of a superheterodyne receiver embodying one form of the invention,

Fig.2 illustrates a modification of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the different figures correspond to similar circuit elements, there is shown in. Fig. 1 an automatic volume control arrangement fora" superheterodyne receiver.. It is to be clearly understood that the nature of the receiver with which the volume control: network is operatively associated is of comparatively little importance insofar as the present application is concerned. The superheterodyne'type of receiver has been employed as an illustration, because it isat the present time substantially universally employed in broadcast reception in the United States and in -many foreign countries. Additionally, the superheterodyne type of receiver is illustrated by virtue of the factthat the alternating current voltage source for the gain control device, hereinafter to be described, may be thexlocal oscillator network of a superheterodyne receiver. For this reason the various networks of the receiving arrangement shown in Fig. 1 are conventionally represented. The numeral 1 designates an intermediate frequency amplifier tube whose resonant input circuit 2 is tuned'to the operating intermediate frequency of the receiver.

The input circuit 2 is reactively coupled to the intermediate frequency circuit 3, which is also tuned to the operating intermediate frequency. Those skilled in the art will understand that the circuit 3 may be coupled to the output electrode of the usual first detector network, and that the first detector network will be operatively associated with a tunable local oscillator circuit. The collected signals would, of course, be impressed upon'the input circuit of the first detector, and the first detector input circuit is tunable overthe operating signal frequency range. These various networks preceding the intermediate frequency" circuit 3 are very well known, and neednot be described in any further detail. The receiving system can be of the type employed as a portable receiver in an automobile, or in a cabinet in the home of the user. The operating range of the receiver can be in the broadcast range, or the receiver may be of the multi-range varietyand in that case it can be operated in thevarious short wave bands outside the broadcast band.

The amplifier 1 includes in its grounded cathode lead the usual self-bias network 4 which comprises a resistor shunted by capacity. The

numeral 5 denotes the resonant output circuit of I the amplifier I, and each of circuits 5 and 6 .is tuned to the operating intermediate frequency. Circuit 6 is the input circuit of the combined detector and AVC rectifier tube (AVC designating automatic volume control). The signal output electrode 8 of tube 1 includes in circuit therewith the load resistor 9 across which the useful audio voltage is developed, the condenser l by-passing radio frequency components to ground. The audio voltage developed across resistor 9 is transmitted by coupling network II to the subsequent audio frequency utilization network, and the latter may comprise one or more. stages of audio frequency amplification. The final audio amplifier stage can be followed by any desired type of reproducing device.

The tube is of the 6K8 type. Such type of tube is Well known to those skilled in the art, and comprises a triode section and a hexode section having a common cathode. The triode section comprises the cathode l2, grid l3 and plate I4, while the hexode section comprises cathode I2 and the grids |5, |6, l1 and I8 arranged successively between the cathode and the audio output electrode 8. The cathode l2 is connected to ground through the self-bias resistor I9, the latter being shunted bya radio frequency by-pass condenser 20. The Voltage drop developed across resistor l9 comprises the normal negative bias for grids l3, l5 and H which are connected in common to the high potential side of the detector input circuit 6. Grids l6 and I8 are connected in common to a desired source of positive potential, and may be established at a direct current voltage of +100 volts. It is pointed out that f the output electrode 8 may be established at a high positive potential. The low potential side of input circuit 6 is established at ground potential, and the triode section plate M'is connected to ground through a resistor 2|. The alternating current voltage source is connected between the cathode l2 and plate M by virtue of a connection to ground and another connection through condenser 22 to the plate end of load resistor 2|. The uni-directional voltage developed across resistor 2|, due to the rectification of the alternating current voltage applied through condenser 22, is utilized as a gain control voltage.

Thus, the plate end of resistor 2| is connected to the signal input grid of amplifier I through a filter comprising resistor 23 and condenser 24. The resistor 23 and condenser 24 are connected in series across load resistor 2|, and the junction of the filter elements is connected by lead 25 and filter resistor 25 to the low potential end of the input coil of circuit 2. It will, therefore, be seen that the signal input electrode of amplifier I, in the absence of a signal carrier amplitude at circuit 6 which is above a predetermined minimum value, possesses a negative grid bias determined by the network 4. However, as the signal carrier amplitude at circuit 6 increases above the minimum value the voltage developed across resistor 2| is added to the normal negative bias of the input grid of amplifier and causes the gain of amplifier to be reduced. The automatic volume control arrangement is utilized in the receiver, as is well known at the present time, automatically to vary one ,or more of the high fre. quency amplifiers preceding the detector input circuit in such a manner as to maintain a sub-' stantially constant carrier amplitude level at the input circuit of the audio demodulator. Because of fading phenomena, or other causes which result in ffects analogous to fading, the signal car-' rier amplitude level at the audio demodulator tends to fluctuate. The automatic volume control arrangement reduces his tendency, and for this reason is utilized in the receiver.

The self-bias network |920 has its elements sufiiciently large so that the hexode section operates as a cathode bias detector. Of course, the hexode could be operated as a fixed bias detector with cathode |2 grounded, and grids l5 and I1 returned to a fixed negative bias voltage. The alternating current voltage, from whatever source derived, is applied to the triode section plate I4 through the condenser 22, as stated previously.

During a negative half cycle of the alternating current voltage plate |4 draws no current. During the positive half cycle of the alternating current voltage the triode section draws an amount of plate current which depends on the average voltage of the triode section grid l3 during this positive half cycle. 'If a strong radio frequency signal potential is impressed on grid l3, then the triode section plate current will be large. The direct potential of the triode plate-|4 will adjust itself to a value such that the amount of positive charge flowing out of condenser 22 through the triode on positive half cycles of alternating current is equal to the amount of positive charge flowing onto condenser 22 through resistor 2| on negative half cycles.

It follows that an increase of signal to grid I3 causes the direct potential of the plate M to become more negative. This is the reason that the uni-directional value existing at the plate end of resistor 2| is filtered and used as automatic volume control bias. A convenient source of the alternating current voltage for the triode section plate M in many modern radio receivers is the cathode of the full wave rectifier employed in the receiver power supply network. The frequency of the alternating current at the cathode ofthe full wave power rectifier is cycles, which is, of course, easier to filter than 60 cycles. If the single section filter 23-24 is not sufiicient, it will be understood that two or more filter sections can be employed. 7

Another possible source of alternating voltage to be applied at condenser 22 is the local oscillator network of the modern superheterodyne broadcast receiver. It is not believed necessary to show in detail the specific connections between condenser 22 and the power supply rectifier, 01' the tunable local oscillator tank circuit of the superheterodyne receiver, since those skilled in the art are fully aware of the manner of coupling load resistor 2| to either of these alternating voltage sources. It is pointed out that sufficient voltage from the local oscillatorcircuit may be obtained in a superheterodyne receiver to provide sufficient automatic volume control bias for the pre-modulator stages. The lead 25 is designated as AVC to denote that this is the path over which the gain control bias is transmitted to the gain control electrodes of the various signal transmission tubes preceding the detector stage.

- It will be noted that the tube 1 functions simultaneously as an audio detector and as an AVC rectifier. The signal carrier energy is not only applied to each of control grids I5 and H], but is also applied to the grid I! located between the positive screen grids |-6 and I8.

In Fig. 2 there is shown a modification of the invention wherein the tube 1 is replaced by a tube 30 of the pentode type. This tube has its cathode 3| shown connected to ground, while the signal input grid 32 receives its negative bias from a fixed bias source designated by the symbol -C which is connected to the low potential end of the input circuit 2. The positive screen grid 33 is located between the negative signal grid 32 and the suppressor grid 34. The output electrode, or plate, 35 is connected to a desired positive potential source through the load resistor 36, and the audio voltage developed thereacross is transmitted to the following audio frequency utilization network, as desired above. The suppressor electrode 34 functions as the cold electrode of the rectifier which comprises cathode 3| and the electrode 34. In this case the alternating current voltage is applied to suppressor electrode 34 through the condenser 40, and it is pointed out that the source of alternating voltage is the receiver power rectifier. As explained above the alternating voltage source may consist of the local oscillator network, if desired. The numeral 50 denotes the load resistor which develops the AVG voltage, and the resistor 58 is connected between suppressor electrode 34 and ground. The series filter section l52 is connected in shunt across the load resistor 53, and the junction of the filter elements is connected to the AVG lead 60.

If alternating voltage from the oscillator tank circuit is utilized the radio frequency filtering provided in the plate circuit of the detector tube 38 will serve to keep the local oscillator voltage out of the audio stages. On the other hand, if 60 cycle, or 120 cycle, voltage is used from the power rectifier network there may occur a certain amount of hum in the audio output circuit of the detector tube. A simple way to prevent this hum from becoming objectionable is to design the audio stages and loudspeaker so that they have a relatively low response at frequencies in the region of 60-120 cycles. It will, therefore, be seen that the pentode tube 33, which may be of the 6J7 type if desired, is connected to function as a biased detector with signal carrier energy applied to the input grid 32, whereas the alternating voltage is applied to the suppressor electrode 34. The latter cooperates with cathode 3| and grid 32 to provide a signal-controlled diode rectifier of the local alternating voltage for producing gain control bias.

It is pointed out that where the alternating voltage source is the oscillator tank circuit of a superheterodyne receiver, the added load on the oscillator tank circuit may be an advantage in some cases. This follows from the fact that an increase in local oscillation amplitude would cause a comparatively large increase in AVC bias, because of the increase in amplitude of the alternating voltage applied to the AVG rectifier cold electrode. This arrangement would make the sensitivity of a receiver independent of oscillator amplitude over a certain range of variation. The latter observation, of course, applies with equal force to either of the modifications shown in Figs.

electrodes adapted to receive electrons therefrom and an auxiliary control electrode disposed in the electron stream to said pair of cold electrodes, a signal input circuit connected between the auxiliary electrode and the cathode, means providing a negative bias for the control electrode thereby to render the cathode, auxiliary control electrode and one of said pair of cold electrodes operative as a biased detector, means coupled solely between and to said cathode and the second'cold electrode,'and independent of said one cold electrode, for applying an alternating voltage between the cathode and said second of said pair of cold electrodes, and a load impedance connected between the cathode and said second cold electrode thereby to develop a uni-directional voltage thereacross which is derived from the rectified alternating voltage.

2. In combination with an electron discharge tube comprising at least a cathode, a pair of cold electrodes adapted to receive electrons therefrom and an auxiliary control electrode disposed in the electron stream to said pair of cold electrodes, a signal input circuit connected between the auxiliary electrode and the cathode, means providing a negative bias for the control electrode thereby to render the cathode, auxiliary control electrode and one of said pair of cold electrodes operative as a biased detector, means coupled solely between and to said cathode and the second cold electrode, and independent of said one cold electrode, for applying an alternating voltage between thecathode and said secondof said pair of cold electrodes, a load impedance connected between the cathode and said second cold electrode thereby to develop a uni-directional voltage thereacross which is derived from the rectified alternating voltage, and means connected to said load impedance for utilizing the uni-directional voltage to maintain the carrier amplitude at the detector signal input circuit substantially uniform 3. In combination with a tube of the type including a cathode, a plate and at least two grids arranged in succession therebetween, a signal carrier input circuit connected between the cathode and the first grid, an audio output circuit connected between the plate and cathode, an alternating voltage source coupled solely between and to the cathode and the second grid and independent of said plate circuit, and means for deriving from the resulting rectified alternating voltage a uni-directional voltage whose magnitude is directly dependent on the amplitude level of the signal carrier energy at said first grid.

4. In combination with a tube of the type including a cathode, a plate and at least two grids arranged in succession therebetween, a signal carrier input circuit connected between the cathode and the first grid, an audio output circuit connected between the plate and cathode, an alternating voltage source coupled solely between and to the cathode and the second grid and independent of said plate circuit, means for deriving from the resulting rectified alternating voltage a uni-directional voltage whose magnitude is directly dependent on the amplitude level of the signal carrier energy at said first grid, and an automatic gain control connection to said deriving means for utilizing said uni-directional voltage.

5. In combination with an electron discharge tube of the type comprising independent triode and hexode sections, a signal input circuit connected to an input grid of each of said sections, an audio output circuit connected to the output electrode of said hexod'e section, an. alternating voltage: source coupled between the cathode and output electrode of said triode section, and means for utilizing the uni-directional voltage derived from: the rectified alternating voltage in said triode section.

6. In combination with an electron discharge tube of the type comprising a triode section and a hexode section having a common cathode electrode, each of said sections including an input electrode, a signal input circuit connected to: the input electrode of each section, each of said sections having an output electrode, an audio voltage output circuit connected to the output electrode of said hexode section, a uni-directional voltage output circuit connected to the output electrode of the triode section, an alternating current voltage source, and means for impressing the alternating voltage between said common cathode and said output electrode of the triode section.

'71. In combination with an electron discharge tubeof the type comprising a triode section and a hexode section having a. common cathode electrode, each of said sections including an input electrode, a. signal input circuit connected to the input electrode of each section, each of said sections having an output electrode, an audio voltage output circuit connected to the output electrode of said hexode section, a unidirectional voltage output circuit connected to the output electrode of the triode section, an alternating current voltage source, means for impressing the alternating voltage between said common cathode and saidou-tput electrode of the triode section, a signal transmission tube coupled to the said input circuit, and means for regulating the gain of said: signal transmission tube in response to the amplitude of the said uni-directional voltage.

8'. In a radio receiving system of the type comprising asi'gnaltransmission tube and a succeeding detectorstage, said detector stage including an. electron discharge tube which comprises a cathode and at least two output electrodes, a signal input circuit. coupled to the output of said signal: transmission tube, an audio voltage output circuit coupled to one of said output electrodes, an alternating voltage source of a frequency substantially difierent from the signal input circuit frequency coupled solely between and to said cathode and the second output electrode, said source being a portion of the system' other than the circuit of said one output electrode, a load impedance connected between the cathode and said second output electrode for developing thereacross a uni-directional voltage which is derived from rectified alternating voltage from said source, means for applying the unidirectional. voltage to said signal transmission tube to control the gain thereof, and means connected to said signal input circuit for controlling the electron flow to each of said output electrodes thereby toproduce audio voltage in said audio voltage output circuit and to vary the magnitude of said. uni-directional voltage across said load impedance.

HERBERT M. NEUSTADT. 

